The present invention relates to a combined treatment process for inhibiting deposition and corrosion in a cooling water system and the formation of gypsum scale in a flue gas desulfurization (FGD) wet scrubber.
An open recirculating cooling water system consists of primarily a cooling tower and a heat exchanger. Heat from a process, such as the condensing of a steam, is transferred via the heat exchanger into a fluid medium which is usually water. This heated water is pumped to the top of a cooling tower and falls as droplets through a gaseous medium, usually air, to the basin below. Air is drawn up through the cooling tower which contacts the falling water droplets, and cools the water by sensible heat transfer and evaporation. The cooled water is returned to the heat exchanger, to pick up more heat from the process.
Because water is evaporated in the cooling tower and the fresh water entering the cooling water system contains impurities such as silt and dissolved mineral solids (dissolved metal ions such as calcium, magnesium, and iron, and anions such as carbonate, phosphate, silica, and sulfate), these impurities become concentrated in the recirculating cooling water. The calcium and other metal ions can come out of solution and from deposits on the heat exchangers in the form of a scale such as calcium carbonate. This happens because the ions become concentrated and exceed solution solubility in the cooling water. The silt can also stick to the surfaces of the heat exchanger. These deposits impede heat transfer across the heat exchanger negatively impacting the efficiency of the cooling process.
The concentration of the solution anions also effects corrosion. Specifically, high levels of chloride or sulfate ions increase the corrosion potential of the metal portions of the cooling system such as the heat exchanger, transfer lines and pumps. Corrosion is further accelerated by deposits such as mentioned above. Therefore, scale and corrosion inhibitors are added to the cooling system to control corrosion and scaling.
FGD wet scrubbers are designed to remove sulfur dioxide (SO.sub.2) from flue gas by contacting the flue gas with reagent solution or slurry in an absorber. The absorber is typically of spray tower design, where the recirculating slurry droplets, usually from 2 to 4 levels of spray nozzles, contact the SO.sub.2 countercurrently, and the clean, scrubbed flue gas goes off the stack, after passing a mist eliminator.
The function of the mist eliminator is to entrap mist particles in the gas stream upon impingement. Mist eliminators are usually made of plastic baffles, chevrons, or honeycombs. Deposit buildup on the mist eliminator increases flue gas velocity and pressure drop across the mist eliminator. Eventually, the induced-draft (ID) fan becomes unable to move flue gas through at the desired rate, requiring FGD modules to be shutdown for cleaning.
Most FGD wet scrubbing processes add limestone (CaCO.sub.3) or lime (CaO) to make up a reagent solution slurry. These reagents react with the SO.sub.2 in the flue gas to produce a solids slurry containing calcium sulfite hemihydrate and calcium sulfate respectively, the latter being present as gypsum. Sulfur oxides are precipitated out of the reagent solution and removed from the system via a bleed line to eventual dewatering and landfill or sale as a raw material.
Scaling caused by calcium suflite hemihydrate and gypsum deposition has been observed in or on the absorber, mist eliminators, dewatering thickeners, transfer and circulation lines, valves, and pump parts of FGD units. Calcium sulfite scale is typically soft and generally may be removed by either reducing slurry operating pH or hydroblasting the unit with water. Gypsum scale is much harder and is very difficult to remove.
Gypsum scale is more prominent in FGD systems that employ limestone rather than lime. Limestone has a lower solubility than lime and a larger amount of excess limestone reagent is required for sulfur dioxide removal. The use of excess limestone slurry causes a high carryover from the absorber to the mist eliminator. The solid phase of the slurry droplets tends to be deposited; whereas; the liquid portion gathers into a large droplet and falls back into the slurry reaction tank. Since a vast number of FGD wet scrubbers currently in operation utilize limestone reagent, widespread gypsum scaling problems exist. To control gypsum scaling, various gypsum scale inhibitors are added to the FGD mist eliminator wash water.
Due to water balance limitations, it is common to employ cooling water system blowdown as a makeup stream for FGD wet scrubber wash water. In doing so, however, an operator would have to employ two different chemical inhibitor treatment programs. The first controls cooling water system deposits and corrosion, and the second controls FGD wet scrubber gypsum scaling. Accordingly, it is necessary to employ and maintain separate feed lines for different inhibitors, one specific to cooling water and the other specific to FGD mist eliminators.
Dual treatment programs are more expensive because they require additional maintenance, pumps, piping, chemical inventory, and the like. A single treatment program which inhibits deposition and corrosion in cooling water and gypsum scaling in FGD systems would decrease expenses, increase system efficiency and be accordingly welcomed by those skilled in the art.
It is therefore an object of the present invention to provide a combined process for inhibiting water system scaling and corrosion in cooling water systems and gypsum scaling in FGD systems.
It is further an object of the present invention to provide an inhibitor formulation which inhibits corrosion and calcium phosphate, calcium carbonate, and silt deposition in cooling systems, and gypsum scaling in FGD systems. Other objectives which are apparent from the specification are also contemplated.